This invention relates generally to monolithic microwave integrated circuits and more particularly to circuits which provide balanced to unbalanced transitions.
As is known in the art, a balun is a circuit which provides a balanced to unbalanced transition or vice versa. A balun generally has four ports, a common port, two branch ports, and an isolated port which is usually shorted or terminated in a resistance for isolation. As a type of transformer, two of such baluns can be used in place of a pair of transformers in a broadband microwave doubly balance mixer. An input balun ideally produces in response to a single input signal two equal magnitude output signals which differ from each other by 180.degree. of phase, whereas an output balun combines two input signals with a 180.degree. phase difference to produce a single output signal.
Passive baluns which are known in the art include planar and non-planar baluns. Non-planar baluns, however, are most often used to construct doubly balanced mixers because most planar balun circuits such as hybrids are very limited in bandwidth. Examples of non-planar baluns include a strip conductor disposed over a dielectric substrate having underlying said strip conductor a tapered ground plane conductor. A pair of such sections when placed between a diode ring will provide a double balanced mixer, generally referred to as a ring mixer. A star mixer uses a second type of non-planar balun which includes an input r.f. strip conductor and an input L.O. strip conductor bridged over a diode mixer and spaced from a common patterned microstrip ground plane conductor. The problems with these particular approaches is that they are not easily fabricated using monolithic microwave integrated circuit techniques, since MMIC fabrication is a planar process and such approaches are non-planar in nature.
One technique of providing planar or MMIC compatible balanced mixer circuits other than the limited bandwidth hybrid approach is to simulate a center tap transformer by using center tap, double wound, spiral or rectangular microstrip inductors. Such elements at microwave frequencies, however, have relatively low values of Q (i.e. lossy elements), which increase overall mixer noise figure. Moreover, isolation, magnitude, and phase characteristics of such mixers degrade at microwave frequencies particularly above X-band.
Another approach to a MMIC balanced mixer is an extension of the r.f. push-pull amplifier as shown in a paper entitled "A Monolithic GaAs FET R.F. Signal of Generation Chip" by Van Tuyl, ISSCC Digest of Technical Papers Feb. 14, 1980, pp. 118-119. Such a circuit uses the inherent 180.degree. phase shift produced by a field effect transistor to create two 180.degree. out of phase signals which then are used to drive a push-pull amplifier stage that acts as a mixer when a local oscillator signal is provided as a gain modulating control. Since the above described circuit uses active devices such as the field effect transistors to produce balanced signals, the circuit provides overall gain and noise performance characteristics which are acceptable at low r.f. frequencies. However, at microwave frequencies where the physical dimensions of various electrical connections between the active devices become a significant fraction of a wavelength, the balanced performance quickly degrades with increasing frequency. Thus, it is difficult for such a circuit to provide good isolation or balanced operation above a few GHz.